Currently, communication systems are evolving into next-generation mobile communication systems that provide high-speed multimedia services. The next-generation mobile communication systems use multiple access schemes proposed to make efficient use of their limited resources to provide high-speed multimedia services.
SDMA is one of the typical multiple access schemes. In a communication system using SDMA (hereinafter referred to as an “SDMA communication system”), when a Base Station (BS) uses a plurality of antennas, areas which are orthogonal to each other may be formed; the number of areas corresponds to the number of the antennas that also are orthogonal to each other. Thus, signals from Mobile Stations (MSs) located in different areas are removed by orthogonality between beams in antenna beam patterns during their transmission/reception, so the signals do not interfere with each other.
Meanwhile, the BS of the SDMA communication system can transmit data only to a small number of MSs for a given time. The BS selects MSs satisfying a semi-orthogonal criterion from among the MSs located in its service zone, as MSs to which it will simultaneously transmit data in the same time period. The semi-orthogonal criterion may include a value used for determining whether each MS is in an orthogonal state, a Signal to Noise Ratio (SNR) threshold, and the like. For example, the value used for detecting the orthogonal state can be CSI.
A description will now be made of an operation in which the BS chooses the MSs to which it simultaneously transmits data in the same time period.
The BS receives CSI that is fed back from each of multiple MSs, and selects MSs in their semi-orthogonal state by comparing the received CSIs with the semi-orthogonal criterion. In other words, the BS compares CSIs fed back from multiple MSs with a predetermined reference CSI, and chooses MSs that have fed back CSI being greater than or equal to the reference CSI, as MSs in a semi-orthogonal state.
In addition, a scheduling method by which the BS receives CSIs from MSs can be roughly classified into a periodic scheduling method and a dynamic scheduling method. In the periodic scheduling method, the BS receives CSIs fed back from MSs only in a specific period of a downlink (DL) frame. In the dynamic scheduling method, the BS sends a CSI feedback request to MSs whenever the need arises.
Regarding the periodic scheduling method, since the BS receives CSIs from MSs only in a predetermined time period, the MSs may suffer from an increase in CSI feedback delay, and the number of MSs from which the BS can receive CSIs may be limited undesirably. Also, as to the dynamic scheduling method, a downlink control signal needed for the CSI feedback request acts as overhead.
As described above, when using SDMA, the BS must receive all CSIs from multiple MSs in the same time period, in order to choose MSs in a semi-orthogonal state. The BS selects semi-orthogonal MSs by comparing the received CSIs with the reference CSI, thus causing an increase in computation.